NRC Chairwoman Allison Macfarlane travels to California for the first time since taking office in July to visit the plant and meet with elected officials, environmentalists and union representatives.

Allison Macfarlane, second from right, head of the U.S. Nuclear Regulatory… (Eliot Brenner, Associated…)

The chief of the U.S. Nuclear Regulatory Commission made her first visit Monday to the troubled San Onofre nuclear plant, offering assurance that the facility will remain closed until its safety is certain but giving few hints about its ultimate fate.

NRC Chairwoman Allison Macfarlane traveled to California for the first time since taking office in July to visit the closed plant and meet with elected officials, environmentalists and plant workers' union representatives.

The plant has been shut for nearly a year because of unexpected deterioration of tubes in its four giant steam generators, replaced less than two years earlier. Officials learned of the unusual wear after a tube sprang a leak last January, releasing a small amount of radioactive steam.

The NRC is reviewing a restart proposal that plant operator Southern California Edison submitted for one of the plant's two reactor units. The proposal entails running that unit at 70% capacity for five months in hopes that operating at reduced power will alleviate the conditions that caused the tubes to vibrate excessively and rub against support structures and adjacent tubes.

Activists have criticized the proposal as an "experiment" with the public's safety. Macfarlane reiterated assurances that the commission would not sign off on a restart plan unless convinced that the plant can be operated safely.

"We aren't doing this as an experiment. We would want to make sure that the plant can operate safely, period," Macfarlane said after touring the plant, on the coast just south of San Clemente.

The commission has set up a team based at its regional headquarters in Texas to deal with the San Onofre issue. Commission staff may make a decision as early as March on the restart proposal for the plant's Unit 2.

Meanwhile, there are separate proceedings before the NRC regarding a petition by environmental group Friends of the Earth.

The group alleges that Edison should have gone through a license amendment process for all design changes made when it replaced the steam generators and should be required to get a license amendment — which would include a courtroom-like public hearing process — before the NRC makes a decision on the restart proposal. Macfarlane declined to comment on that issue.

In another separate but related proceeding, the California Public Utilities Commission is investigating the cost to ratepayers from the problems at San Onofre and could eventually decide to refund money to customers who have continued paying the costs of the plant during the extended outage.

Macfarlane was also slated to visit Diablo Canyon, the state's other nuclear plant.